1. Technical Field
This invention relates to an image forming apparatus, such as a copier, a facsimile, a printer, a multifunction device prepared by combining these devices, etc., and a method of arranging a sheet detector implemented in the image forming apparatus.
2. Related Art
A so-called image forming apparatus, such as a digital copier, a laser printer, etc., that employs electrophotography generally forms an image by transferring a toner image onto a recording medium such as a recording sheet, etc., and fixing the toner image onto the recording medium while pressing and heating the toner image under prescribed conditions.
In such an image forming apparatus, heat and pressure or the like must be adjusted to appropriately fix the toner image. Especially when a high-quality image is to be formed on the recording medium, a condition of fixing the toner image needs to be set differently depending the type of recording medium because the quality of the image printed on the recording medium is greatly influenced by the material, thickness, humidity, smoothness, and coating or the like of the recording medium, where coating generally means coating and printing with ink or paint and the like. The smoothness is represented by the time it takes (in seconds) for a prescribed quantity of air to flow through a gap formed between the sheet and a testing board tightly contacting each other.
Here, a smoothness and fixability of the recording medium (e.g., a sheet) are highly correlated to each other. This is because, a fixing rate of toner changes in a recess of irregularities of the recording medium depending on a degree of the irregularities. Therefore, not only a low-quality image is obtained, but also an unusual image such as defective fixing, etc., is produced in some cases if the smoothness is neglected in fixing the toner image.
Meanwhile, as image forming apparatuses have gotten better and as methods of representation have become more diverse, there now exists hundreds of different types of recording sheets (e.g., recording media). That is, there is a wide range of brands of each type of recording sheet, with different basis weight and a thickness or the like. Therefore, to form a high-quality image, fixing conditions need to be set in detail in accordance with the type of sheet and the brand or the like of the recording medium.
For example, as the type of recording medium, a plain paper sheet, a coated paper sheet, such as a gross coated paper sheet, a mat coated paper sheet, an art coated paper sheet, etc., an OHP sheet, and a special sheet prepared by embossing a surface of a sheet, etc., are exemplified. The special sheet is increasingly utilized in recent years. Furthermore, there exists media other than the recording sheet.
With current image forming apparatuses, it is extremely common to set fixing conditions in accordance with a basis weight of the recording sheet. Typically, recording media are classified by basis weight, such that a sheet having a basic weight of from about 60 g/m2 to about 90 g/m2 is a plain paper sheet, a sheet having a basic weight of from about 91 g/m2 to about 105 g/m2 is a medium cardboard, and a sheet having a basic weight of from about 106 g/m2 to about 200 g/m2 is a cardboard and so on. Subsequently, fixing temperature and conveying speed of the recording medium are varied by classification.
The basic weight of the recording medium is generally listed on the package of the recording medium or the like, thereby enabling a user to ascertain the basis weight easily. This information is important because generally the user needs to set the above-described basis weight information by himself or herself, thus adding an extra step to preparation for printing. In addition, if the user erroneously sets the above-described basis weight information, an intended high-quality image cannot be obtained. Accordingly, an image forming apparatus capable of detecting a thickness of a recording medium with a sensor and automatically selecting a given recording medium has been researched in the past.
By contrast, since the smoothness of the recording medium is not usually printed on the package, it is extremely hard for a user to ascertain the smoothness by himself or herself. For this reason, the user is forced to use a sensor, etc., to obtain the smoothness of the recording medium.
Moreover, as mentioned earlier, the smoothness is highly correlated to the fixability. However, the smoothness is hard to detect within a short time period because the smoothness represents the time required for a prescribed quantity of air to flow through a gap formed between a sheet and a testing board contacting each other. Therefore, in the past, a sensor capable of detecting an amount of surface roughness or quantity of reflected light as alternative characteristics to smoothness has been employed because of the high correlation between surface roughness or quantity of light on the one hand and to the smoothness on the other.
As a known system to detect the smoothness of the recording medium, a light ray emitted from a light source such as a light-emitting diode (LED) is emitted onto a recording medium, and quantity of light reflected back from the recording medium is detected. According to the above-described reflection light system, since the smoothness can be detected without contacting the recording medium, the recording medium is not damaged.
As a known smoothness detecting method based on this type of a light reflecting system, quantity of light emitted and reflected in a regular reflecting direction from a surface of the recording medium is detected, and a smoothness thereof is calculated based on the detected luminous energy. Another known system includes multiple luminous energy detectors to detect not only quantity of light emitted and reflected from a surface of the recording medium in a regular reflective direction but also quantity of diffused light as well in order to identify the smoothness of the recording medium based on these two quantities of light.
The smoothness detected in these detecting systems is used in setting image forming and fixing conditions such as fixing temperature, etc.
A given time is generally needed from when image formation is started to when either an image is transferred onto a recording medium or the fixing temperature practically reaches a target level thereof. Accordingly, in an image forming apparatus that utilizes a detected smoothness of a recording medium in setting fixing and image forming conditions, the smoothness needs to be detected considering the above described given time as needed. Accordingly, positioning and detection timing of the smoothness detector are particularly important.
However, in the above-described image forming apparatuses, neither optimum positioning nor detection timing of the smoothness detector is determined considering a prescribed time period from when image formation is started to when either an image is transferred onto a recording medium or fixing temperature practically reaches a target thereof.
Some image forming apparatuses include a sheet thickness detector to detect a thickness of a recording medium, etc., and change one or more fixing conditions in accordance with the thickness of the recording medium. Even in these image forming apparatuses, however, to appropriately set a fixing condition, positioning and detecting timing of the sheet thickness detector are particularly important.